It is already known to use light guides to illuminate panels for general lighting purposes and for display applications (e.g. for illuminating signs and advertisements, and also for illuminating liquid crystal displays). More recently, it has also been proposed that light guides could be used as the basis for signal lamps, for example for the head or tail lights of automotive vehicles, to replace the conventional signal lamps that typically comprise an incandescent bulb and a reflector together with a lens system for shaping and directing the lamp output beam. Conventional signal lamps of that type are comparatively inefficient and bulky, and are often not aesthetically pleasing. In addition, the bulb position is usually visible through the lens system resulting, in use, in “hot spots” and a generally non-uniform appearance. It has been suggested that some, at least, of those disadvantages could be overcome through the use of light guides.
An additional reason for the increasing interest in light guides as the basis for signal lamps is that they make it possible to use light-emitting diodes (LEDs), or other solid state light-emitting devices as light sources. Light-emitting diodes are becoming increasingly reliable and available at lower costs, and offer particular advantages as light sources because they take up only a small amount of space and are highly efficient in comparison with incandescent sources.
The application of light guides and diode lasers to the signal lamps of automotive vehicles is discussed in an article entitled “Lighting—New Technology, New Techniques” by T. Fohl, E. R. Freniere and J. T. Remillard that appeared in the August 2001 edition of “Optics & Photonics News”. In addition, U.S. Pat. No. 5,700,078 describes a vehicle lighting system comprising a fibre optic light guide coupled, at one end, to a laser light source and, at the other end, to an optical element which forms, for example, a tail lamp of the vehicle. The optical element comprises a transparent, solid sheet of plastic through which light from the fibre optic light guide is guided by total internal reflection. Reflective surfaces in the optical element cause light to be directed out of the sheet, across the front surface thereof. Further modifications of that type of system, which enable the front surface area (or “footprint”) of the optical element to be reduced, are described in U.S. Pat. Nos. 5,857,770; 5,791,757; 5,890,796 and 6,036,340. In each case, the optical element (i.e. the solid sheet of plastic) that forms the vehicle signal lamp is, effectively, an edge-lit solid light guide (i.e. a solid light-guiding plate into which light is directed in a direction generally parallel to a major surface of the plate from an elongated light source or a similar arrangement located adjacent an edge of the plate).
Other lamps formed from edge-lit solid light guides and suitable for use in vehicles are described in U.S. Pat. Nos. 5,101,325; 5,165,772 and 5,375,043. The problem of improving the appearance of vehicle lamps that are illuminated by a back light source is addressed in U.S. Pat. No. 5,684,633.
Illuminated panels based on hollow light guides are also known and have been proposed for use as display panels and for illuminating liquid crystal displays (see, for example, EP0377309; GB2 310 525; and an Application Bulletin entitled “Thin Light Box” issued in March 1990 by Minnesota Mining and Manufacturing Company of St. Paul, Minn., USA). In addition, U.S. Pat. No. 6,080,467 describes an illuminated sign comprising a light box, the interior surfaces of which comprise a multi-layer reflective optical film. Hollow light guides would appear to offer advantages for applications that require the weight of the light guide to be kept as low as possible but, despite that, solid light guides have typically been more widely used, because they are comparatively simple to produce and are the easiest way of transporting light.
U.S. Pat. No. 6,078,421 describes various backlight arrangements using substantially monochromatic light, particularly for illuminating a liquid crystal display. The backlight arrangements make use of an interference filter, comprising a stack of dielectric layers on a transparent substrate, in combination with a diffuse reflector to provide a collimated beam of monochromatic light. Luminaires that have a similar construction are described in U.S. Pat. Nos. 4,161,015 and 4,649,462.
An illuminated sign, suitable for use on an automotive vehicle, is described in WO 00/65277. The sign comprises a housing having diffusely-reflecting interior surfaces and a front sign face through which light from inside the housing is transmitted, the light being supplied by a light fibre located on an interior surface of the housing.